We have obtained a suitable vacuum solution for Dirac fields in FLRW spacetimes with a Yukawa interaction with a classical scalar field. This solution is a good approximation when the classical scalar field value dominates the dynamics of the system, e.g. the beginning of the reheating era. We have computed the adiabatic expansion for the stress-energy tensor and the two point function and, by making use of the \(\mu \)  parametrization of the adiabatic regularization we have fixed the renormalized stress-energy tensor to vanish at some initial time \(t_0\) . This is somewhat similar to the instantaneous vacuum for scalar fields in FLRW spacetimes proposed in [1]}. We have also checked its consistency for several inflationary models .
